Throughout the course of its (at minimum)
6-year lifetime, there is a series of maneuvers planned for Terra. These
maneuvers fall into two categories: (1) attitude maneuvers that involve changing
the orientation of the spacecraft in any of three possible axes (called roll,
pitch, and yaw); and (2) orbital maneuvers that involve changing either the
orbital altitude or the orbital plane of the satellite in order to adjust the
local time at which Terra crosses the equator.

There are "thrusters" positioned strategically at locations on the
spacecraft that allow the flight operations team to adjust the orbital position
of Terrato fly it, essentially. Onboard tanks contain hydrazine, a
reactive gas commonly used on spacecraft. In firing the thrusters, the ground
operations team is actually releasing some hydrazine from the tank and routing it
over a heated catalyst that breaks down the gas into products that are forced out
through small rocket engines, thereby providing thrust for the spacecraft.

Throughout its lifetime, Terra will periodically perform a variety of
attitude maneuvers for calibration purposes. In these maneuvers, Terra changes
its orientation with respect to the Earth. The spacecraft will pivot on one or
more of three possible axes to enable its sensors to view the blackness of deep
space or the moon. Spinning on its X-axis is called a roll; spinning on its
Y-axis is a pitch; and spinning on its Z-axis is a yaw maneuver. (Image by
Reto Stockli)

The first major maneuver planned for Terra, on January 11, was to raise it
from its lower elliptical orbit to a higher, circular orbit of 705 km above the
Earth. The flight operations team had to meet the requirement of flying in
formation with Landsat 7 so that Terra descends across the equator at roughly
10:30 a.m. local time, about 15 minutes behind Landsat 7 on the same flight path.
To do this orbital maneuver, the Terra team had to fire Terras onboard
thrusters to both raise its orbit as well as keep the satellite properly oriented
with respect to the Earth. At 6:19 p.m. EST, the team began firing the
thrusters. Moments later Terras flight computer aborted the maneuver when
it correctly diagnosed that Terra was beginning to roll more than expected.

"I was in the mission control room and was growing very concerned about
the increasing roll rate," Quinn recounts. "At the time, I didnt
have an explanation of why Terra was rolling." Quinns mind raced as
he watched Terra enter "safe mode." There are two separate protective
programs built into the spacecraft to limit how much error is allowed in its
orientation. If any error limits are exceeded, then the thrusters stop firing
automatically and the protective programs take over to re-establish Terra into an
Earth-pointing orientation. In this way, if the ground controls fail for any
reason, the sensors will by default be pointed at the Earth and the
missions science objectives can still continue safely.

Again, Grady scrambled his flight operations team into a brainstorming session
that lasted, off and on, for a couple of weeks. Why did Terra begin an unplanned
roll when they started boosting its orbit? After many days filled with long
hours of exhaustive work examining all the possibilities, and creating and
modifying computer models of the spacecrafts behavior, the team narrowed
the list of possibilities to a few key suspects.

Gradys team determined that the roll was caused by a combination of two
things: (1) a misalignment in the satellites center of gravity with respect
to the thruster settings, and (2) plumes from two of the thrusters impinging on
the solar array. "There is a thruster pairing matrix that
resides in the software system aboard the spacecraft that describes how much
torque you get from each thruster," Grady elaborates. "When the
control system needs a certain amount of torque, it uses that matrix to decide
which thrusters to fire to get the desired thrust. We discovered that these
thruster pairing matrices werent exactly correct for the center of gravity
of the spacecraft, but we knew wed have to fine-tune the matrices on
orbit."

What Grady and his team hadnt known is that the gas molecules escaping
the thrusters on the rear of the spacecraft would spread out into a cone shape so
that enough of these molecules would impact the solar panel with an effective
force of approximately one-quarter of a pound. (View an animation [6.2MB] of exhaust gasses
hitting Terra's solar panel) "At 30 feet long, what you
basically have with the solar array is a very long moment arm, or lever, on the
spacecraft," he explains. "Even though Terra weighs approximately
11,000 pounds, the solar panel gives you such a long lever that it doesnt
take much force on the end of it to roll the spacecraft."

After days of analyzing telemetry data and conducting simulations that nearly
identically matched Terras behavior, Grady and his team were convinced that
they understood the problem well enough to resume the ascent maneuver. This
time, they did two things to avoid another unplanned roll: (1) they fine-tuned
the thruster pairing matrices to more precisely account for the satellites
center of gravity and external disturbances generated by the thruster plumes, and
(2) they placed the solar array into a position where it would minimize any
impingement from thruster plumes. They began the ascent with a series of short
thruster bursts and once they knew they had solved the problem, they did a series
of longer thrusts.

Terra reached its target orbital altitude on February 23 and its sensors began
opening their aperture doors the next day for science operations. Terras
activation had begun.

During the first
maneuver to raise Terra into its final orbit the spacecraft rolled excessively. Using computer simulations, the mission controllers
discovered that exhaust from a rocket thruster was pushing on Terra's large solar panel. (Image and animation by Reto Stöckli [6.2MB])